1. Field of the Invention
The invention relates to curable silane-grafted isomonoolefin/vinyl aromatic copolymers particularly useful in sealants, caulks, adhesives and coating compositions.
2. Description of Related Art
Curable copolymers comprising at least one alphamonoolefin and styrene or para-alkylstyrene are known in the prior art. For example, U.S. Pat. No. 5,162,445 discloses curable random copolymers containing a C.sub.4 to C.sub.7 isomonoolefin, e.g., isobutylene, and para-alkylstyrene, e.g., para-methylstyrene prepared by cationic polymerization of the monomer mixture in the presence of a Lewis acid catalyst, followed by halogenation, e.g., bromination, of the resultant copolymer to introduce benzylic halogen into the polymer molecules. The resultant halogenated copolymer is curable in a variety of ways, including by means of zinc oxide or promoted zinc oxide curing systems normally used to cure halogenated butyl rubber. These halogenated copolymers may also be further functionalized by reaction with a nucleophilic reagent capable of displacing benzylic halogen to provide copolymers containing ester, hydroxyl, carboxy, nitrile, quaternary ammonium, mercapto and other functionality as described in the aforementioned U.S. Pat. No. 5,162,445.
Analogous copolymers prepared by copolymerization of C.sub.2 to C.sub.12 monoolefin, e.g., ethylene or propylene, and para-alkylstyrene, e.g., para-methylstyrene, using a metallocene catalyst, and functionalized derivatives thereof, are disclosed in U.S. Pat. No. 5,543,484.
Copolymers such as described above exhibit many outstanding properties after curing, such as improved resistance to oils and greases, resistance to oxygen and ozone degradation, excellent impermeability to air, water vapor and many organic solvents as well as resistance to aging and sunlight. These properties render these materials, particularly curable elastomeric copolymers where the olefin component comprises at least 50 mole % of C.sub.4 to C.sub.7 isomonoolefin such as isobutylene, as ideal candidates for applications such as sealants, caulking and coatings. However, in many of these applications, particularly outdoor applications, the material must be in pliable form for use and must be curable in-situ under ambient conditions without the need to apply excessive amounts of heat.
It is known in the prior art that amorphous polyolefins such as copolymers of ethylene and propylene can be rendered cross-linkable by reaction of the polyolefin with a free radical generating compound and an unsaturated hydrolyzable silane, as disclosed in U.S. Pat. No. 3,646,155. However, such materials usually have a relatively high glass transition temperature and are not sufficiently pliable at ambient temperature to serve as polymeric components in sealant or caulking compositions.
Attempts to apply such modification techniques to polyisobutylene-based polymers have not been successful because polyisobutylene-based polymers tend to degrade in the presence of free radical initiators. An attempt to overcome this problem is disclosed in U.S. Pat. No. 4,245,060, which describes silane-functionalized terpolymers of isobutylene, alkylstyrene and a third monomer which is an unsaturated cyclic diene, e.g., cyclopentadiene. In U.S. Pat. No. 4,524,187, copolymers of isobutylene and a triene containing two conjugate double bonds are silylated to provide cross-linkable compositions. In each of these cases, polymer modification takes place by the addition of a silylradical to residual double bonds present in the polymer molecules; however, the presence of residual unsaturation in the polymer molecules severely detracts from the normally good chemical and heat stability of polyisobutylene polymers.
Other methods for introducing silane functionality into binary copolymers of isobutylene and an alkylstyrene are disclosed in U.S Pat. No. 5,426,167, which suggests the reaction of brorninated isobutylene/para-methylstyrene copolymers with an N,N-dimethyl-3-aminopropylsilane. This reaction is a nucleophilic displacement of bromide ion by the tertiary amine and is thus a two-step functionalization process involving both halogenation and nucleophilic displacement reactions.